[SagarikaGhosh]

Microorganisms employed to enhance the availability of nutrients, viz., nitrogen (by fixing atmospheric nitrogen) and phosphorus (by solubilizing phosphorous), to the crops are called biofertilizers. Of these, biological nitrogen fixation offers an economically attractive and ecologically sound route for augmenting nutrient supply. Thus various species of Rhizobium for legumes, blue green algae (BGA) or cyanobacteria ( Aulosria, Nostoc, Anabaena etc) and Azolla ( a fern containing symbiotic nitrogen fixing BGA) for wet land rice, and Azotobacter/Azospirillum for several crops can play significant role in agriculture. On the other hand, the phosphatic biofertilizers, including the bacteria, Thiobacillus, Bacillus, etc. and the mycorrhizal fungus Glomus help in increasing the solubility/availability of nutrient P, which is already present in the soil in sparingly soluble forms. These microorganisms, however, do not bring in P from outside. Therefore, in a way, they deplete the soil P reserves but considering the low utilization efficiency of phosphatic fertilizers, P-solubilizing biofertilizers can play an important role in improving the efficiency of utilization of P residues left in the soil.

List of some important microorganisms with practical/potential application as biofertilizers:
Organism Activity Association Use in crops

Rhizobium N2- fixation Symbiotic Legumes

Azospirillum N2- fixation Associative Graminaceous crops

Azotobacter N2- fixation Asymbiotic Wheat, rice

Blue Green algae N2- fixation Asymbiotic Rice

Azolla- Anabaena N2- fixation Symbiotic Rice

P-solubilizing bacteria P solubilization Asymbiotic Many crops

Mycorrhiza P solubilization Associative Many crops


Criteria for selection of microorganism for biofertilizer (e.g. Nitrogen Fixer):

1. Ability to fix dinitrogen over a range of environmental conditions

2. Ability to compete with other strains

3. Ability to multiply in broth and survive in the carrier

4. Ability to survive in seed pellets

5. Ability to persist in soil

6. Ability to form nodules

7. Ability to continue to fix atmospheric dinitrogen in presence of soil nitrogen

8. Ability to migrate in soil and colonize it

9. Ability to be stable during storage.